Low velocity zero temperature coefficient acoustic surface wave delay line

ABSTRACT

An acoustic surface wave delay line having a single crystal tellurium dioxide substrate member the acoustic surface wave propagation surface of which substantially coincides with a plane defined by the Euler angles Lambda 0*, Mu 90*, and Theta 39*.

United States Patent [191 Slobo'dnik, Jr. Nov. 13, 1973 LOW VELOCITY ZERO TEMPERATURE [56] References Cited COEFFICIENT ACOUSTIC SURFACE WAVE UNITED STATES PATENTS DELAY LINE 3,202,846 8/1965 Ballato et al 3l0/9.7 [75] Inventor: Andrew J. Slobodnik, Jr., 3,699,482 10/1972 Ash et al 3l0/9.7

Burlington, Mass. [73] Assignee: The United States of America as Primary P Rolinec represented by the secretary of the Assistant Examiner-Marvin Nussbaum Air Farce, Washington, Dc Attrn eyHarry A. Herbert, Jr. et al.

[22] Filed: Dec. 15, 1972 21 A l [57] ABSTRACT N 315 746 1 pp 0 An acoustic surface wave delay line having a single crystal tellurium dioxide substrate member the acous- Cl 3 333/72, 3 10/95 tic surface wave propagation surface of which substan- Cl H03h 9/00, H0311 /3 tially coincides with a plane defined by the Euler an- [58] Field of Search 333/ R, 72; l La bda 0, Mu and Theta 39.

1 Claim, 5 Drawing Figures LOW VELOCITY ZERO TEMPERATURE COEFFICIENT ACOUSTIC SURFACE WAVE DELAY LINE BACKGROUND OF THE INVENTION This invention relates to acoustic surface wave delay lines, and in particular to such devices having both ultra low acoustic surface wave velocities and zero temperature coefficients.

Surface wave acoustic devices are currently coming into widespread systems use for the performance of a variety of delay and signal processing functions. However, for many applications it is highly desirable to use a temperature compensated cut to support the surface waves; that is, a crystalline orientation having zero temperature coefficient of delay. In fact, a review of the current state of the art indicates that a limitation on the application of surface wave encoders and decodersto multiple-access, secure communications sytems, is the degradation of the peak-to-sidelobe ratio of the auto correlation function due to temperature differences.

The only temperature compensated cuts presently known are on a quartz. The most widely used is the ST- cut, X-propagating orientation. Unfortunately, crystalline quartz possesses a moderately high surface wave velocity which leads to undesirably long substrates for any devices constructed using this material.

An acoustic surface wave delay line that exhibits both ultra-low velocity and zero temperature coefficient characteristics is disclosed in my copending patent application, Ser. No. 315,739, entitled Low Velocity, Zero Temperature Coefficient Acoustic Surface Wave Delay Line Having Group And Phase Velocity Vector Coincidence, filed on even date herewith. Although the delay line disclosed thereby is a low velocity zero temperature coefficient device having the added advantage of group and phase velocity coincidence it requires special techniques for achieving effective transducer coupling in most applications.

There currently existstherefore the need for ultra low velocity, zero temperature coefficient acoustic surface wave delay lines having reasonably high piezoelectric coupling constants. The present invention is directed toward satisfying this need. I

SUMMARY OF THE INVENTION This invention utilizes acoustic surface wave propagation on a propagating surface that coincides with particular crystalline axes of tellurium dioxide to provide room temperature zero temperature coefficient delay lines and matched filters adapted to use in very small spaces. The propagating surface is defined by a phase that substantially coincides with Euler angles Lambda Mu 90, and Theta 39. RF energy is converted to acoustic energy by an electromagnetic wave to acoustic surface wave input transducer. The acoustic energy propagates on the tellurium dioxide surface and is finally reconverted to RF energy by an acoustic surface wave to electromagnetic wave outputtransducer. The very low velocity of the particular propagating surface orientation used allows very long delay time for a given crystal size. The zero temperature coefficient of delay characteristics possessed by tellurium dioxide permits these delay lines and filters to be used in many systems that require RF time delay and signal processing without costly and bulky ovens and temperature feedback control.

It is a principal object of the invention to provide a new and improved acoustic surface wave delay line.

It is another object of the invention to provide acoustic surface wave delay lines and filters having both ultra low acoustic surface wave velocities and zero temperature coefficients of delay.

It is another object of the invention to provide small lightweight delay devices that are capable of effecting long RF delays and that do not require temperature stabilization ovens or feedback controls.

It is another object of the invention to provide an acoustic surface wave delay line having, simultaneously, ultra long delay time, zero temperature coefficient of delay and a reasonably high piezoelectric coupling constant.

These, together with other objects, features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the illustrative embodiment of the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a tellurium dioxide crystal substrate member cut to the particular crystalline orientation comprehended by the invention;

FIGS. 2a, 2b, and 2c illustrate the coordinate system used to define acoustic surface wave propagation in terms of Euler angles; and,

FIG. 3 is a plan view of an acoustic surface wave delay line as comprehended by the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there is illustrated thereby a single crystal tellurium dioxide substrate member 4 having an acoustic surface wave propagating surface 1 1 cut in accordance with the principles of the present invention. Such a cut must substantially conform to a plane defined by Euler angles Lambda 0", Mu 90 and Theta 39.

The coordinate system used to define acoustic surface wave propagation in terms of Euler angles is illustrated by FIGS. 2a, 2b, and 2c. The phase velocity vector lies along the 1 axis while the plate normal lies along the negative 3 axis. The crystalline axes are given by X, Y and Z while the Euler angles are Lambda, Mu and Theta. FIGS. 2a, 2b and 2!: illustrate the standard starting coordinate system in which the propagation axes line up with the crystalline X, Y and Z axes. It follows therefore that the standard Eular angle rotation Lamdba 0, Mu Theta= 39 specified above refefs to rotation in the X 2 plane st arting Win12; propagation direction along the X axis and a plate normal along the Y axis.

An acoustic surface wave delay line incorporating the principles of the present invention is illustrated by FIG. 3. It comprises substrate member 4, electromagnetic wave to acoustic surface wave input transducer 5 and acoustic surface wave to electromagnetic wave output transducer 6. Substrate member 4 is fabricated of single crystal tellurium dioxide and has its propagation surface 11 cut to conform with the above designated Euler angles. Input transducer 5 consists of interdigital fingers 7 and 8 which may be affixed to the propagating surface 11 by standard photolighographic techniques. Output transducer 6, consisting of interdigital fingers 9 and 10 is similarly affixed to propagation surface 11. Tellurium dioxide with the particular propagation surface cut comprehended by the invention has a coupling parameter AV/ 0.00008. Accordingly it is also intended that other state of the art means for launching and detecting acoustic surface waves be an integral part of the invention. These include the combination of interdigital transducer and thin piezoelectric films, bulk to surface wave conversion means, wedge techniques and other coupling enhancing devices. The geometry, dimensions and relative positions of the transducer are determined by the operating frequency, delay time requirement and other parameters of the particular device specified. The distance between the launching and receiving structures determines the delay time according to the formula:

delay time (seconds) distance (meters)/l424 where 1424 meter/second is the surface wave velocity. The particular orientation of the propagation surface comprehended by the present invention causes the phase velocity vector to differ from the group velocity vector by 37.5. Accordingly output transducer 6 must be offset from input transducer by 37.5 as illustrated in FIG. 3. The delay line of the present invention can be utilized in filter applications by using coded transducers. This can be accomplished by varying finger widths and spacings, by inverting appropriate fingers, and by amplitude and frequency weighting.

While the invention has been described is one presently preferred embodiment, it is understood that the words which have been used are words of description rather than words of limitation and that changes within the purview of the appended claims may be made without departing from the scope and spirit of the invention in its broader aspects.

What is claimed is:

1. An acoustic surface wave delay line comprising a single crystal tellurium dioxide substrate member having a propagation surface defined by a plane that substantially coincides with the Euler angles Lambda 0, Mu and Theta 39, an electromagnetic wave to acoustic surface wave input transducer disposed on said propragation surface, and an acoustic surface wave to electromagnetic wave output transducer disposed on said propagation surface. 

